This invention relates generally to voltage protection and more particularly to a triggered voltage suppression network which provides a low voltage, clamp of transient surges, the clamping voltage level thereof determined by a clamping device, and a breakdown voltage level at which the network first reacts to a transient independently determined by a crowbar device.
A wide variety of electrical devices exist which are susceptible to damage from voltage sources having transient surges. Transient surges generally arise from a number of sources such as lightning strikes, inductive machines and switching and powering of electrical equipment. Unfortunately, such transient surges are often unpredictable and utility companies cannot adequately guard against hem especially when they are localized as is the case of a transient produced by a neighbor's switching on a motor. To further complicate matters, most electrical equipment in use today has inadequate surge suppression incorporated therein.
Devices and circuit configurations which afford some degree of voltage protection against transients on AC lines are known in the art. Illustrative of such are voltage clamping devices and crowbar devices. However, there are disadvantages associated with each of these which render them less than adequate in many applications.
Known voltage clamping devices such as varistors clamp a supply voltage to a specific voltage level upon an increase in current and/or voltage through/across the clamping device beyond a specific amount. An increase in supply current generally accompanies an increase in supply voltage, and vice versa. The specific voltage level to which the supply voltages is initially clamped is known as the clamping voltage. However, as the current through the clamping device is further increased, the voltage across the (clamping device increases beyond the initially clamped voltage, although at a much lower rate. Unfortunately, if the voltage source impedance is relatively low, the clamping device will be less effective due to its own dynamic impedance and the device may not be able to adequately suppress the transient. A most significant disadvantage of known clamping devices arises due to the practical requirement that their clamping voltage be set substantially higher than the anticipated maximum peak line voltage in order to provide a reasonable margin of safety. By way of illustration, the breakdown voltage associated with known clamping devices used on 120 VAC line is typically 250 VDC at 1mA. During sure this voltage will increase proportionally to the current that the transient is delivering. A voltage of 500VDC and more may be reached on a 8/20 us, 3000A waveform, resulting in a stress to the equipment that is being protected.
Known crowbar devices such as gas tubes and thyristors and the like involve a switching action such as that associated with the breakown of a gas between electrodes in a gas tube, or the turning on of a thyristor. Unlike clamping devices, once crowbar devices are activated, they enter a very low impedance state which diverts transients from the parallel connected load. Unfortunately, such crowbar devices suffer from hysteresis. More specifically, although activated at a transient voltage in excess of the supply voltage, crowbar devices are not deactivated until the voltage drops to a level sufficiently far below the supply voltage. In other words, where power from a steady-state voltage source follows a transient surge discharge, such "follow-current" is often too high to turn the crowbar device off. Furthermore, crowbar devices have a tendency of not recovering even after zero-voltage crossing. An additional disadvantage of crowbar devices lies in their low impedance state when activated. Although it is desired to protect equipment connected to a 120VAC supply from transients, it is preferable to provide a voltage as near as possible to 120VAC during the disturbance and not to crowbar the supply voltage towards zero volts, thus effectively disconnecting the load equipment from the power supply.
A transient surge suppression network whose breakdown voltage level may be chosen independently of its clamping voltage, and whose clamping voltage may be chosen near to the anticipated peak supply voltage, is thus clearly desirable.